1. Field of the Invention
The present invention relates to a spark plug for use as an ignition source of an internal combustion engine, and more particularly to a semi surface discharge type spark plug having a structure that the igniting surface of a ground electrode is disposed opposite to the outer surface of a central electrode.
2. Description of the Related Art
Hitherto, a semi surface discharge type spark plug having a structure shown in FIGS. 14 and 15A to 15C is known. FIG. 14 is a partial cross sectional view of the semi surface discharge type spark plug. FIG. 15A is a cross sectional view showing a leading end portion (a spark discharge portion) of the semi surface discharge type spark plug shown in FIG. 14. FIG. 15B is a diagram showing a different-diameter portion (a gap) formed between a leading end 24e of an elongated leg portion 24 shown in FIG. 15A and an outer surface 12a of a central electrode 12. FIG. 15C is a diagram showing the thickness of the leading end 24e of the elongated leg portion 24 shown in FIG. 15A.
Note that description will now be made such that the lower portion shown in FIG. 14 is the leading end portion and the upper portion is the rear end portion.
The semi surface discharge type spark plug 10 is provided with an insulating member 20 made of alumina or the like. The insulating member 20 incorporates a corrugation portion 22 formed in the rear end portion thereof and an elongated leg portion 24 formed in the front end portion and formed into a pyramidal shape. The insulating member 20 has an axial hole 26 formed along a central axis 18 of the insulating member 20. A terminal 13 is accommodated in a rear end portion in the axial hole 26. The rear end of the terminal 13 projects over the rear end of the corrugation portion 22. The central electrode 12 is, through a glass resistance 11, accommodated in the axial hole 26 at a position adjacent to the terminal 13. The central electrode 12 is formed into a rod shape and made of an alloy mainly composed of nickel. A front surface 12f of the central electrode 12 projects over the leading end of the elongated leg portion 24 of the insulating member 20.
The leading end of the insulating member 20 is accommodated in a main metal shell 14 formed into a cylindrical portion. A leading end 24e of the elongated leg portion 24 projects over an opened front surface 14c of the main metal shell 14. A packing member 17 is disposed between the rear end of the elongated leg portion 24 and the main metal shell 14. A male thread portion 14a arranged to be screwed in a female thread portion provided for a cylinder head of an engine is formed around the leading end of the main metal shell 14. A base portion 16b of each of ground electrodes 16 is secured to a front surface 14c of the main metal shell 14.
Each of the ground electrodes 16 is bent into an L-like shape facing the central axis 18. An igniting surface 16a at the leading end of each of the ground electrodes 16 is disposed opposite to the outer surface 12a of the central electrode 12 so that an igniting portion SG is formed between the igniting surface 16a and the outer surface 12a (see FIG. 15A). As shown in FIG. 15A, a first gap g1 is formed between the outer surface 12a of the central electrode 12 and an igniting surface 16a of the ground electrodes 16. A second gap g2 is formed between the outer surface of the leading end 24e of the elongated leg portion 24 and the igniting surface 16a.
As shown in FIG. 14, a hexagonal portion 14b is formed at the rear end of the main metal shell 14 to permit a tool, such as a plug wrench, to be fit to the hexagonal portion 14b when the male thread portion 14a is screwed in a female portion provided for the cylinder head of the engine.
The thermal expansion coefficient is different between the central electrode 12 made of metal and the insulating member 20 made of alumina ceramic. Therefore, there is difference in the thermal expansion between the two elements. To prevent a fracture of the insulating member 20, a different-diameter portion (a gap) 15 is formed between the outer surface 12b of the central electrode 12 and the axial hole 26, as shown in FIG. 15B.
As shown in FIG. 15C, an intersection is formed between an extension line 60a drawn by outwards extending a line 60 indicating a side surface 24f of the elongated leg portion 24 adjacent to the igniting portion and an extension line 61a drawn by extending a line 61 indicating a side surface 24c of the elongated leg portion 24 toward the side surface 24f of the igniting portion. The distance (hereinafter called a "thickness") tp from the intersection to a line 65 indicating the inner surface of the axial hole 26 is 1.1 mm. Gap ga of the second gap g2 is 0.5 mm. Length (the axial directional distance from the side surface 24f of the elongated leg portion 24 to a sealing surface 24g to which the packing member 17 is joined, as shown in FIG. 14) L of the elongated leg portion 24 is 12 mm. The difference (hereinafter called the "difference .DELTA..phi.d in the diameter") between diameter .phi.d.sub.1 of the central electrode 12 and diameter .phi.d.sub.2 of the axial hole 26 is 0.09 mm. Distance .DELTA..phi.d/2 of the different-diameter portion 15 is 0.09 mm/2=0.045 mm.
The male thread portion 14a of the main metal shell 14 is screwed in the female portion of the cylinder head. Thus, the semi surface discharge type spark plug 10 structured as described above is joined to the cylinder head such that the ground electrodes 16, the leading end 24e of the elongated leg portion 24 and the leading end of the central electrode 12 are exposed to the inside portion of the combustion chamber of the engine. Then, a high electric resistance cable is connected to the terminal 13. When discharge voltage is applied, a spark is ignited between the igniting surface 16a of the ground electrodes 16 and the central electrode 12. Thus, mixture in the combustion chamber is ignited.
The cleanability of a spark made by the spark plug for an internal combustion engine will now be described with reference to FIG. 16 which shows the principle of the cleanability.
As shown in FIG. 16, the discharge voltage is applied such that the central electrode 12 has negative polarity and the ground electrodes 16 has positive polarity. Therefore, the elongated leg portion 24 is charged with the positive polarity owing to dielectric polarization. Hence it follows that negatively-charged particles contained in the spark made at the end 12g of the central electrode 12 is attracted to the side surface 24f of the elongated leg portion 24. Therefore, the negatively-charged particles reach the igniting surface 16a of the ground electrodes 16 through a discharge passage formed along the side surface 24f of the elongated leg portion 24, as indicated with symbol S shown in FIG. 16.
Therefore, conductive fouling substances allowed to adhere to the side surface 24f of the elongated leg portion 24 owing to fouling are burnt out by the spark.
That is, fouling resistance of the semi surface discharge type spark plug having the above-mentioned spark cleanability is superior to that of an aerial discharge spark plug.
Problems to be Solved by the Invention
When sparks frequently move along the side surface 24f of the elongated leg portion 24, the energy of the spark causes the side surface 24f to be consumed. Thus, a groove 24k (see FIG. 16) is sometimes formed. That is, so-called "channeling" sometimes occurs. When channeling proceeds, a through portion is formed in a portion of the elongated leg portion 24 adjacent to the discharge passage S. As a result, a fracture of the elongated leg portion 24 occurs or heat resistance deteriorates. That is, there is apprehension that the durability of the spark plug deteriorates.
When metal oxides or the like produced owing to metal powder made owing to the consumption and owing to combustion are introduced and deposited in the different-diameter portion 15 (see FIG. 15B), the different-diameter portion 15 is clogged. When heat cycles in great temperature difference are rapidly repeated in the above-mentioned state, the difference between the thermal expansion of the elongated leg portion 24 and that of the central electrode 12 sometimes causes the elongated leg portion 24 to be broken. That is, there is apprehension that the durability of the spark plug deteriorates.
When the distance of the different-diameter portion 15 is enlarged, heat reduction becomes unsatisfactory. Thus, pre-ignition occurs, that is, heat resistance deteriorates. When an attempt to improve the heat reduction is made, the length L of the elongated leg portion 24 must be shortened.
In recent years, enlargement of the output of the engine causes a requirement for a spark plug having improved durability to arise.